The present invention relates to epitaxial reactors and, more particularly, to a gas collector for epitaxial reactors.
Continuing advances in the semiconductor industry have resulted in the development of highly complex thin-film deposition processes for fabricating semiconductor devices that are packaged for use in the manufacture of sophisticated electronic devices. The thin films of material that are deposited on the semiconductor wafers are often referred to as epitaxial layers. High speed electronic transistors, quantum-well diode lasers, light-emitting diodes, photodetectors, and optical modulators incorporate structures composed of numerous epitaxial layers ranging in thickness from several microns to as thin as a few tenths of a nanometer. These epitaxial layers are typically deposited, or grown, on a single-crystal substrate, i.e., the semiconductor wafer.
One method of forming epitaxial layers on a semiconductor wafer is known as chemical vapor deposition (CVD). In a typical manufacturing process of a wafer, for example, silicon or gallium arsenide in extremely pure crystalline form is overlaid sequentially with numerous layers of materials, which function as conductors, semiconductors, or insulators. Each subsequent layer is ordered and patterned such that the sequence of layers forms a complex array of electronic circuitry. The semiconductor wafer can then be subsequently cut along predetermined scribe lines into individual devices, commonly referred to as xe2x80x9cchips.xe2x80x9d These chips ultimately function as key components in electronic devices ranging from simple toys to complex supercomputers.
CVD processes normally take place within a reaction chamber. Initially, the semiconductor wafer is placed within a reaction chamber containing an inert atmosphere, and the temperature within the reaction chamber is elevated. Reaction gasses containing the compound or element to be deposited are then introduced to react with the surface of the semiconductor wafer, which results in deposition of the required film onto the semiconductor wafer. The reacted gasses are continually introduced and removed from the reaction chamber until a requisite film thickness has been achieved.
An example of an epitaxial reactor is described in U.S. Pat. No. 4,961,399, to Frijlink, which is incorporated herein by reference. This patent describes a reactor into which reaction gasses are introduced via a quartz funnel that is located at the center of the reactor. The reaction gasses then flow radially outward towards a quartz ring that bounds the reactor. Along the circumference of the quartz ring are equidistant slits, which collect the reacted gasses. Bounding the upper portion of the reaction chamber is a quartz disk. The quartz disk seals against O-rings, witch are positioned behind the quartz ring. Because quartz is a brittle and inflexible material, the quartz disk does not seal against the quartz ring. Instead, a gap is provided between the quartz disk and the quartz ring to prevent chipping of either.
This gap between the quartz disk and the quartz ring can cause problems within the reactor. For example, reaction gasses can escape through the gap and can form deposits outside the reaction chamber, and these deposits can interfere with the working of the reaction chamber and can also flake off and act as contaminants. Although a narrower gap can be provided, if a hard foreign body wider than the gap is introduced into the gap, such as during the opening of the reaction chamber, the foreign body could prevent the quartz disk from sealing properly over the reaction chamber or can cause chipping of either the quartz disk or the quartz ring.
An attempted solution to the above-described problems is disclosed in U.S. Pat. No. 4,976,217 to Frijlink, which is incorporated herein by reference. This patent describes a collecting crown or gas collector, which is both used to collect reaction gasses from the reaction chamber and also to provide a seal between the reaction chamber and a quartz disk or cover.
The gas collector and reaction chamber of the prior art is illustrated in FIGS. 1 and 2. The gas collector 1 is mounted on a supporting platform 4 by a horizontal plate 10 that rests upon the supporting platform 4. The supporting platform 4 is typically formed from quartz and is positioned within a cylindrical body 19 of the reactor that surrounds the reaction chamber and the gas collector 1. The cover 8 of the reaction chamber bounds the top of the reaction chamber and seals against the upper ridge 6 of the gas collector 1 and against toric joints 20 within the cylindrical body 19.
The gas collector 1 is further illustrated in FIG. 3. The gas collector 1 is formed from a folded plate of molybdenum having elastic properties. The molybdenum plate is folded along horizontal folding lines 13 and vertical folding lines 14 to form multiple flat plates 17, 5, 18, 9, 3, 10 that are connected to one another along the folding lines 13, 14. Also, two plates 2, 3 are touching without being fixed to each other. The combination of plates 17, 5, 18, 9, 3, 10 form a conduit 30 that encircles the reaction chamber. One of the plates 17 includes regularly spaced inlets holes 12 that collect the reaction gasses from the reaction chamber. Instead of the inlet hole 12, as shown below on the right-hand side of FIG. 3, the wall plate 17 can be provided with folded lower projections 15, which separate the movable lower edge 2 away from the fixed edge 3 to leave a slot between the edges 2, 3 through which the reaction gas can then pass.
The ""217 patent states that an essential element of the gas collector 1 is the vertical baffle plate, which is constituted by plates 17, 3 with the lower edge 2 of the upper plate 17 being pressed with a sliding motion against the upper edge of the lower plate 3. The horizontal plates 10 that are connected to the lower plates 3 serve to place the gas collector 1 on the edge of the platform 4. Furthermore, the top plate 5 is inclined and includes an upper ridge 6.
A problem with the gas collector 1 of the prior art is illustrated in FIG. 4. The epitaxial reactor described in U.S. Pat. No. 4,961,399 was designed under the assumption that an even distribution of gasses flows out from the middle of the reactor to the gas collector 1. The prior art assumed that by regularly placing inlet holes 12 along the front wall 17 of the gas collector 1, the flow distribution (illustrated in FIG. 4 with arrows) of the reaction gasses would also be regularly distributed. However, it has been discovered that the gas collector 1 of the prior art does note produce an even flow distribution.
The flow distribution is skewed by the placement of exhaust tubes 29 in the rear wall 18 of the gas collector 1. Gasses will follow the path of least resistance, and the path of least resistance into and through the gas collector 1 is in a direction generally towards the location of the exhaust tubes 29. In this manner, a greater volume of gas passes through the inlet holes 12 that are immediately adjacent the exhaust tubes 29 as compared to inlet holes 12 that are farther away from the exhaust tubes 29. Therefore, an even flow distribution of reaction gasses into the gas collector 1 is not present.
If the reaction gasses are not flowing evenly from the center of the reaction chamber, the deposition process varies depending upon the location of the wafers within the reaction chamber because the densities of the various constituents of the reaction gasses also vary depending upon their location within the reactor chamber. As such, the thickness and quality of the deposition can vary from one wafer to the next, even within the same batch process. For example, when depositing AlxGaAs using the gas collector 1 of the prior art, the percentage (x) of aluminum being deposited varies not only from one batch of wafers to the next, but also varies within wafers in single batch and also within a single wafer. There is, therefore, a need for a gas collector that solves the problems of the prior art by providing an even flow of reaction gasses through the gas reactor.
This and other needs are met by embodiments of the present invention which provide a gas collector for collecting gasses from within a reaction chamber of a reactor. The gas collector includes a body, in which is defined a conduit, at least one inlet, an outlet, and a seal. The seal cooperates with a removable lid of the reactor to prevent escape of the gasses from the reaction chamber. Also, the inlets direct the gasses from the reaction chamber into the conduit, and the outlet exhausts the gasses from the conduit into an exhaust pipe of the reactor. The gas collector additionally includes means for providing an even flow of gasses through the reaction chamber.
By providing a even flow of gasses through the reaction chamber, the deposition process within the reaction chamber is more consistent. A more consistent deposition process results in the thickness and the quality of the deposits not varying as much between wafers in a single batch, between wafers in different batches, and within a single wafer, as compared to the gas collector of the prior art.
In one aspect of the invention, an even flow of gasses through the reaction chamber is provided by including a baffle positioned within the conduit approximate each of the outlets. In another aspect, the even flow of gasses is provided by dividing the body into equally-sized sectors and having a total intake area of inlets in each of the sectors continually increase in a circumferential direction away from an outlet. The increasing total intake area can be provided by increasing a size of each inlet in a circumferential direction away from the outlet and/or increasing a density of inlets in each sector in a circumferential direction away from the outlet.
In other aspects of the invention, the body can be rigid. Further the body can include a first and second members separate from one another. The body can also include inter-member seals that reduce the flow of the gasses across an interface between a first member and a second member of the body. The inter-member seals are disposed between the first member and the second member and allow movement of the first member relative to the second member. Additionally, the gas collector can include a device for pressing the seal against the lid, such resilient members, which are disposed within the conduit. The body can also include graphite.